1. Field of the Invention
The present invention relates to a device for accurately detecting the end of the usage lifetime of a discharge lamp such as a metal halide lamp or the like that is used as a vehicular headlight. Moreover, the present invention also relates to a discharge lamp actuating device that has a unit for detecting the end of the usage lifetime of the discharge lamp.
2. Description of Related Art
Generally, a high voltage pulse is applied to a discharge lamp, which is charged with a few hundred volts of open circuit voltage, to energize the discharge lamp. The discharge lamp is thereby illuminated by switching from a glow discharge state to an arc discharge state. Because the length of a gap between poles of the discharge lamp increases with increasing usage until the end of its usage lifetime, the voltage required to illuminate the discharge lamp increases while its lamp current decreases. Therefore, it is difficult to maintain the discharge operation of the discharge lamp and thus, eventually, such discharge operation stops and the discharge lamp is unusable. When the discharge lamp flickers out while being energized, ordinary discharge lamp actuating devices like those disclosed in Japanese Patent Laid Open Publication No. Hei-6-119982 and U.S. Pat. No. 5,485,059 automatically perform the energization operation again. Therefore, when the discharge lamp, which is near the end of its usage lifetime, stops discharging and flickers out, the discharge lamp actuating device again energizes the discharge lamp. But the discharge lamp eventually flickers out again thereafter.